This invention relates in general to tubing connectors and rod connectors and in particular to connectors for joining capillary tubes of the type used in gas chromatography equipment and other highly sensitive scientific measuring devices or for joining fiber optic rods.
Tubing couplings or tubing fittings in the past have taken variations of two basic forms of pressure fit, these being created by (1) a compressed ring or (2) a physical distortion of the end of the tubing. In the first instance, an o-ring or compression ring is wedged between mating surfaces of a coupling housing comprising the fitting. As a matter of course, two rather substantially constructed members, one acting as a housing and the other acting as a cap are drawn together. Each of these members can have a tapered surface or equivalent structure which bears down upon the ring deforming it and creating a pressure by the ring against the outside diameter of the tubing. This creates a pressure seal between the coupling members and the tubing. The deformable ring occupies a very short distance along the outer diameter of the tubing so that when it is deformed it concentrates specific pressure forces at a very narrow region of the tubing to create the seal. These forces very often permanently deform the wall of the tubing or at least distort it which effects the flow characteristics through the tubing at that point.
In the second instance of the distortion of the tubing, the end of the tubing is flared for mating with the outside taper of a tapered fitting and for being compressed thereagainst by a cap having a similar tapered surface and which threadably engages its mated coupling part. The flaring of this tubing and the insertion of the flared end into the mating tapered surface creates a protrusion into the tubing passageway which effects the flow characteristics through that section.
With large I.D. tubes the effects are minimal, but with capillary tubes these flow characteristic effects can be very significant. Moreover, with these types of couplings two compression members must be used and the fluid flowing through the coupling comes in contact with metal or other surfaces of the coupling housing.
Spontelli, U.S. Pat. No. 4,076,286, provides a tubing fitting wherein a coupling body 12 has a square shoulder against which the end of the tubing seats to create a smooth continuation for the interior wall or passageway. An annular ring 50 has a convex outer surface 56 which seats against and is drawn up to a tapered shoulder 28 of the coupling body 12 when a coupling nut 34 is drawn upon the coupling body 12 by means of threads 30. This causes a crushing of the annular ring 52 to press and impinge on the outer wall of the tubing. As shown in FIG. 2 of the patent, the wall of the tubing is typically distorted at the pressure points to create the positive seal needed.
Jennings, U.S. Pat. No. 4,035,168, provides an inlet splitter for gas chromatograph. This structure includes a coupling structure for mating to a capillary tube 16. This structure includes an end closure 102 having a bore therethrough and an annular fitting into which an aperture nut 106 mounts. The aperture nut carries a central bore through which the capillary tube 16 passes. A sealing means 108 identical to sealing member 70 is caused to spread and thereby clamp down about the capillary tubing 16 as the aperture nut 106 is screwed into the end closure 102. This sealing member can be constructed of rubber, neopreme or silicon and causes a pinching at a narrow area of the capillary tubing wall to create a seal.
Biermacher, U.S. Pat. No. 4,116,837, provides an in line filter arrangement for a hard pressure liquid chromatography apparatus including couplings for mating two stainless steel capillary tubings. A SWAGELOK (trademark) fitting is used for coupling two one-sixteenth inch stainless steel capillary tubing 14. Tube 14 is provided with a fitting member or cap which is threadably engaged onto the external threads of a nipple 12. Each SWAGELOK fitting includes a sealing member which is distorted by the fitting member as it is tightened down on the nipple, causing it to press with great force at a specific point along the capillary tubing.
The construction and connection of capillary tubing is critical in many applications where used in sensitive measuring devices, and none is more critical than in gas chromatographic equipment. The physical structure of the tubing must be maintained constant throughout so that the dynamic characteristics of the fluid passing therethrough are not drastically effected from turbulation, back pressures, dead spaces or other factors. Often inert material, such as fused silica or glass, is used for capillary tubes on chromatograph equipment. The joining of two such silica or glass tubes, or when used, metal capillary tubing, without detectable detrimental chromatographic effects has been a problem.
The prior art often uses two sealing rings to connect the ends of the capillary columns or tubes within a housing device. This metal housing serves as a splice while introducing both dead volume and flow disruption. It is desirable to provide a capillary tubing coupling which minimizes dead volume, flow disruption and eliminates fluid exposure to coupling housing surfaces.
Moreover, each of these prior art coupling structures provides a coupling seal by pressure forces exerted between the mating surfaces of three elements: the tubing and the seal member on one hand, and the seal member and the housing on the other hand.
An object of the present invention is to provide a capillary butt connector for joining the ends of two capillary tubes in close proximity to one another and for minimizing dead volume between the joined ends thereby minimizing chromatographic analytical readout inaccuracies.
A second object of this invention is to provide such a butt connector where the tubes joined are surrounded by a sealing and support structure which is pliable and extends a sufficient distance from the joined ends.
A further object is to provide such a connector where the sealing member reacts to longitudinal forces on it to expand laterally thereby providing a coupling seal between the seal member and tubes alone and thereby eliminating the absorptive effects of fluid contact with the connector housing.